Oxygen ion transport membrane elements are utilized to separate oxygen from an oxygen containing feed. Such elements are fabricated from ceramics that are capable of conducting oxygen ions at elevated temperature. Oxygen within the feed is ionized on a surface of the membrane designated as the cathode surface to form oxygen ions. The oxygen ions are conducted through the element to an opposite anode side where the oxygen ions recombine to form elemental oxygen. The electrons released from the ions during the formation of elemental oxygen are conducted to the cathode side of the membrane element to ionize the oxygen.
The oxygen ion transport is driven by an oxygen partial pressure differential between the cathode side and the anode side. This pressure differential is created or facilitated with the use of a reactive substance that reacts with permeated oxygen to consume the oxygen at the anode side and thereby to produce a decrease in oxygen partial pressure. The exact mechanism of such a reaction is unknown in that it is not known whether the oxygen ions recombine to form elemental oxygen which in turn reacts with the reactive substance or whether the reactive substance reacts with the oxygen ions.
In addition to facilitating oxygen ion transport the reaction of permeated oxygen with the reactive substance will function to at least contribute to the heating of the membrane to its operating temperature. Further, the reaction can have other purposes, for example, the reaction can oxidize the reactant substance to a desired product such as a synthesis gas. A yet further purpose for the reaction is that the heat liberated from the reaction, in addition to heating the membrane, can also act to heat a heat transfer fluid such as water within a boiler.
The ceramic material forming the oxygen ion transport membrane element can be mixed conducting and therefore capable of transporting both oxygen ions and electrons. Additionally, the ceramic material can also be a dual phase of an ionic conductor and an electronic conductor to transport both oxygen ions and electrons. Triple phase mixtures of mixed conductors, ionic conductors and electronic conductors have been used for such purposes. Mixed conductors are typically formed from perovskites such as lanthanum strontium cobalt iron oxide and typical ionic conductors are yttria stabilized zirconia and gadolinium doped ceria.
As can well be appreciated, a desirable oxygen ion transport membrane element produces a maximum flux of oxygen. It is known that oxygen permeance increases proportionally with decreasing thickness. Hence, oxygen ion transport membrane elements are typically fabricated with a thin, gas tight dense layer. Such dense layers typically have a thickness of less than 0.5 mm and as such, are supported on structural porous supporting layers.
For instance, in U.S. Pat. No. 5,240,480, composite membrane structures are disclosed having a dense layer thickness of 10 microns supported by one or more porous supporting layers having pore diameters that are less than 20 micrometers. The theory behind such a membrane design is to minimize the bulk diffusion resistance through the dense layer by making it as thin as possible.
The problem with such conventional membrane architectures, as have been discussed above, is that such thin dense layers are fragile and the porous supports, by virtue of their porosity, are also fragile so that the resultant oxygen ion transport membrane element is not particularly durable in service.
As will be discussed, the present invention provides an oxygen separation and reaction method utilizing an oxygen ion transport membrane element that is more durable than prior art elements by virtue of the provision of a relatively thick dense layer that acts to structurally support the element.